Groundwater contamination is a growing concern in the agricultural and waste management industry. Seepage losses from animal-waste storages, municipal lagoons, and industrial waste retention ponds including mine tailings retention ponds, are common sources of groundwater pollution. A livestock operation produces large amounts of organic effluent and often is stored on site. Typically, the most feasible and cost effective method to store and or treat organic waste is to contain the waste in a lagoon or retention pond. The bottom of these lagoons is generally made from a semi-impermeable layer comprised of synthetic or clayey soil material. In most cases, the cost of synthetic impermeable liners is more expensive than compacted clay liners and therefore the clay liners are used more often for agricultural waste management applications.
The main concern relating to clay liners is their ability to properly maintain a relatively impermeable barrier between the waste and the surrounding material over a length of time without seepage losses that could impact the groundwater quality in the area. When manure storage lagoons require maintenance or are emptied for organic fertilizing purposes, they are usually agitated to mix the solids that have settled out over a period of time. This agitation process can loosen and partially remove the upper layer of the saturated clay liner, which inevitably causes the deterioration in the liner thickness and a less desirable hydraulic conductivity. The guidelines given by the United States Environmental Protection Agency (EPA) for the thickness of a clay liner is one metre and the hydraulic conductivity (K), cannot be greater than 1×10−7 cm/s.
Estimation of soil hydraulic conductivity at a particular site is challenging and expensive at times in order to produce acceptable results. Measuring the permeability of a saturated clay matrix is important in the design of lagoon liners. The ability to predict the rate of nitrate contaminant transport is vital information for evaluating future lagoon sites. Accurate estimates of the hydraulic conductivity act as a tool for monitoring existing sites.
There are a number of methods that are in use to measure the saturated hydraulic conductivity of soils, but these are typically overly expensive, complicated, time consuming and/or produce false results. A sample can be obtained as a core from the lagoon bottom, after pump out, and laboratory tests can estimate the hydraulic conductivity (K), however this disturbed specimen may not be a true representation of the in situ conditions. Pumping the storage dry would cause shrinkage cracks, which could create macropores and fissures, which will act as preferential flow paths for contaminants. To protect the integrity of the clay liner, manure storages are never completely emptied during pump out and this makes it extremely difficult to obtain core samples from the bottom covered with liquid manure slurry. Getting technicians to retrieve core samples under such conditions will be a difficult and time consuming task. Since the manure storages are never emptied completely it is impossible to locate problems by visual inspection by regulators. Currently, core samples are obtained using drill rigs located on the frozen ice layer above the stored manure. However, this is limited to a small window of time during which the outside temperatures remain below sub-zero temperatures (<−15 degrees Celsius) so that the ice cover remains thick enough to support a drill-rig.
Another method commonly used to determine K is a standard borehole pump test, but this intrusive method disturbs the thin upper organic sludge-clay interface and exposes that portion of liner to an aerobic environment. Chemical reactions and biological interaction with the aerobic environment changes the in situ characteristics of the liner. Also, the borehole pump test is more applicable to highly permeable aquifers. However, because of the low hydraulic conductivity of compacted clay, the conventional borehole tests cannot be used efficiently for the lagoons. It is essential to develop an accurate monitoring tool to determine a clay liners performance over time in order to minimize seepage below retention ponds and lagoons to ensure the safety of rural groundwater aquifers.
As noted above, hydraulic conductivity of soils is determined by a number of techniques, but in practice, there are only three main methods: laboratory tests, field tests and empirical methodologies (Domenico and Schwartz 1997 pg.44). The theory of hydraulic conductivity, Darcy's Law and Horslev's Method is reviewed in detail. An overview of electro-kinetics and how it relates to hydraulic conductivity is provided.
Most methods used to measure hydraulic conductivity are derived from the fluid motion through porous media law known as Darcy's Law. This law was named after a French civil engineer Henry Darcy whose experimental methodology for measuring the rate of flow through a porous medium was published in 1856. Darcy's experiment includes a cylinder of length (L) containing a porous medium with manometers attached at either end to measure the water pressure head as water passes through the column. From this simple experiment, Darcy found that discharge Q, is proportional to the change in head pressure and inversely proportional to the length of the column containing the porous media (Fetter 1994). From these relationships Darcy formulated the Hydraulic conductivity constant K, which estimates the rate of flow through a porous medium per unit hydraulic gradient per unit cross-sectional area. Discharge is expressed in the general form Q=−KA(dh/dl), where Q is the discharge (L3/T) and A is the cross sectional area through which the fluid passes and has units L2. The term (dh/dl) is referred as the hydraulic gradient, which is a ratio of the difference in head (h1−h2) between two points and the length separating them (ΔL). The proportionality coefficient K represents the hydraulic conductivity, which has the same units as velocity L/T. The negative sign represents the movement of a fluid in the direction of decreasing hydraulic head (Fetter 1994). The hydraulic conductivity is dependent on fluid properties such as density and kinematic viscosity as well as properties of the porous medium.
Domenico and Schwartz (1997) express the hydraulic conductivity K, in terms of properties that characterize the fluid (water in this case) and porous medium (sand in this case). This relationship is expressed as
  K  =                    N        ⁢                                  ⁢                  d          2                ⁢                  ρ          w                ⁢        g            μ        =                            k          i                ⁢                  ρ          w                ⁢        g            μ      Where
N=Dimensionless shape factor of the sand particle
ρw=Density of water at a specific temperature (M/L3)
g=Acceleration due to gravity (L/T2)
d=Mean grain diameter (L)
μ=Dynamic viscosity of fluid (M/TL)
ki=Intrinsic permeability of the porous medium.(L2)
The intrinsic permeability ki is a property of the porous medium that is equal to Nd2 in the given relationship. The intrinsic permeability is independent of the fluid properties and therefore is a direct measure of flow resistance through a medium. Given a particular fluid, the higher the permeability of a porous homogeneous medium, the greater the ability to transmit flow.
The hydraulic conductivity can be determined in the laboratory using several different techniques, but these methods lack the characteristics of in situ methods that minimally disturb the soils. Two of the most common methods of determining hydraulic conductivity in the laboratory are Constant Head and Falling Head methods.
The constant head permeameter method delivers a constant supply of fluid to a porous medium to maintain a given pressure head. The hydraulic
  K  =            L      ⁢                          ⁢      Q              H      ⁢                          ⁢      π      ⁢                          ⁢              R        2            conductivity is specified by the relationship:where Q is the volume flow rate defined by the cross sectional area of the tube multiplied by the velocity of the fluid. The constant head permeameter is most suitable for estimating the hydraulic conductivity of coarse sands and gravels because of the high permeability of these materials, while the falling head permeameter is more appropriate for fine silt and clay like soils (Wanielista, Kersten and Eaglin 1997).
The falling head permeameter uses a similar relationship for the discharge Q. The falling rate of the water level in the stand pipe is expressed by:
  Q  =            A      ⁢                          ⁢      v        =          π      ⁢                          ⁢                        r          2                ⁡                  (                                    ⅆ              h                                      ⅆ              l                                )                    Where,
v=falling head velocity.
And Darcy's Law can be applied to the soil column as:
  Q  =      π    ⁢                  ⁢          R      2        ⁢          K      ⁡              (                  H          L                )            
After equating both of these equations and integrating, the hydraulic conductivity for a falling head permeameter is represented by the following relationship:
  K  =                    π        ⁢                                  ⁢                  r          2                ⁢        L                    π        ⁢                                  ⁢                  R          2                ⁢        t              ⁢                  ⁢          ln      ⁡              (                              H            1                                H            2                          )            where H1/H2 is the head ratio of initial to final head at a time t(s).
A flexible wall permeameter is a test chamber that contains a porous medium, which is used for both the constant and falling head methods. There are strict guidelines for laboratory procedures when acquiring and testing a porous material sample and are outlined by the American Society for Testing and Materials (ASTM D 5084–90). The laboratory techniques discussed are standard methods of determining K from small soil samples taken from the field.
Field techniques are more accurate methods for estimating in situ hydraulic conductivity. Small-scale lab tests are not representative of the non-uniformities, which are found in geological deposits under subsurface conditions. Examples of such naturally occurring non-uniformities are macro pores, fissures and small channels including worm and rodent holes. These soil structure abnormalities are very challenging to duplicate in an experimental setting and the results from which therefore are only estimators of in situ hydraulic conductivity.
Daniel (1989) explains that in situ permeameters can be divided into four categories. The first two categories are borehole and porous probe permeameters that are used to measure low permeability soils and the other two are infiltrometers and lysimeters that estimate K for permeable agricultural type soils.
The borehole or augerhole method is one of the most popular site investigative and monitoring practices of estimating hydraulic conductivities for relatively shallow water tables. One of these techniques is called the Hvorslev Method or Slug test method, which drills out a standard borehole and inserts a piezometer. In one variation, the piezometer may be installed into sand and therefore does not require a sand pack around well screen to minimize entry losses.
When the static water level (H) is measured, a unit volume of water or metal slug is either introduced or removed out from the well. If a slug is suddenly introduced, then the water level will rise to the initial falling head Ho. As the head decreases, the time is recorded until the water level returns close to the static level H. Water levels can be measured accurately inside the piezometers with pressure transducers that measure the change in head pressure. The data is then plotted where the natural logarithm of the ratio of H/Ho produces a relatively straight line with respect to time. Hvorslev (1951) developed the relationship between the measured hydrostatic head and the pore pressures in the adjacent soil formation as water flowed into the piezometer. Hvorslev noticed a lag time required to equilibrate the pressure difference assuming that a constant flow is maintained at the initial rate into the piezometer and found that the time lag was inversely proportional to the hydraulic conductivity of the adjacent soil. The following equation relates time lag and K:
      T    o    =      A    FK  where,
To=basic lag time for the head level to fall to 37 percent of the initial water level;
F=shape factor which varies with borehole geometry; and
A=Cross sectional area of the piezometer.
When the time lag is established and the shape factor is identified for a particular piezometer or instrument, the above relationship can be rearranged to solve for the hydraulic conductivity of the adjacent soil.
The borehole in situ methods have several limitations such as high implementation costs, poor estimators of the vertical component of hydraulic conductivity and the role of specific storage Ss is completely ignored (Demir, Z. and Narasimhan, T. N. 1994). The specific storage is the amount of water released or absorbed into storage per unit of volume of a porous medium per unit change in fluid head (Fetter,1994).
A similar borehole method uses the Boutwell Permeameter, which measures both horizontal (Kh) and vertical (Kv) coefficients of permeability. This is an improvement from the previous method, but both methods measure a relatively small volume of soil (<<1 m3) and can take any where from a few days to weeks for silty-clay soils with K<1×10−7 cm/s (Daniel, 1989).
Daniel (1989) describes and summarizes nine state of the art in situ hydraulic conductivity estimation methods and instruments for compacted clay soils and lists the advantages and disadvantages of each method. The common element that plagues all in situ methods of estimating hydraulic conductivity are the errors caused by incomplete saturation of the soils (Daniel 1989).
In the laboratory there have been many efforts to relate measured values of hydraulic conductivities to various properties of porous materials (Domenico, P. A. and Schwartz, F. W. 1997).
Empirical methods are adequate for rough estimations of hydraulic conductivities, but should be used only as theoretical tools rather than for practical design applications.
Electro-osmosis is a mechanism, which induces a fluid to flow through low permeable clayey soils. When electrodes are attached to a column of saturated soil and an electrical potential gradient is applied across the soil sample, the fluid will move from the anode to the cathode. The fluid flow is induced by the electric field applied to the soil sample (Yeung, A. T., Gopinath, Sreekumar, Menon, Rajendra, M., Scott, T. B., Datla, Subbaraju. 1993). Water will move under the influence of an electrical potential gradient and the electro-osmotic flow rate can be expressed by the equation:qe=−Ke∇Ewhere
qe is the electro-osmotic flux (m/s); and
Ke is the electro-osmotic conductivity (m2V×s).
The electrical potential gradient is represented by VE and has units V/m.
Since a hydraulic gradient is induced by exposure to an electric field, a probe or device can be developed to estimate saturated hydraulic conductivity by utilizing electro-osmotic behaviour of various soils.